In current ignition systems, such as those implemented in internal combustion engines, an amount of energy that can be delivered to a spark plug to ignite and combust air fuel mixture in an engine cylinder is limited by size and/or cost of a corresponding coil (ignition coil, transformer, etc.). Accordingly, a primary winding of the coil must be sized such that it can store sufficient energy for facilitating both ignition (e.g., spark initiation) and combustion (burning) of the air fuel mixture in an associated cylinder of the engine. For a conventional coil, a large number of primary winding turns are used in order to provide sufficient inductance to store energy for each ignition cycle. Further, in order to achieve a turns ratio that reduces voltage stress on the primary winding, a large number of secondary winding turns can also be used. As a result, a resistance of the secondary winding of such a coil can be in the range of 4-10 kilo-ohm £kohm), which can limit the amount of energy that is delivered to a corresponding spark plug during a spark/ignition cycle (e.g., to ignite and combust fuel and air mixture). Furthermore, energy that is dissipated by a leakage inductance of the coil through a high voltage switch used to control charging of the primary winding of the coil (e.g., an insulated-gate bipolar transistor (IGBT) device), can put electrical stress on the switch (e.g., IGBT device) and also reduce electrical efficiency of the ignition system (circuit).
As an example, current ignition systems (circuits) can include, for each cylinder of an associated engine, an ignition coil, an ignition IGBT device, a control circuit and a spark plug. Such systems can also include an engine control unit (ECU) that communicates with the circuit components for each cylinder to indicate when each cylinder should perform a spark event (ignition event, combustion event, etc.). For example, for a given cylinder, the ECU can provide a command signal (e.g., a logic high level) that causes the control circuit to generate a turn-on voltage for the ignition IGBT. Turning on the ignition IGBT causes current to flow through the primary winding of the ignition coil to store energy for the spark event, where current through the primary winding of the ignition coil increases based on the coil's primary impedance (e.g., inductance and/or resistance).
In such circuits, the coil's secondary side is an open circuit before arcing of the spark plug (e.g., due to the high impedance of the spark plug gap), thus energy (all energy, substantially all energy) for the spark event (ignition and combustion) is temporarily stored in the magnetic core of the coil. To fire the spark plug, the command signal from the ECU can, for this example, change to a logic low level, which results in the ignition IGBT being turned off. This rapid change of current in the primary winding of the coil induces a high voltage spike across the ignition IGBT as the coil's leakage inductance is discharged, and a high voltage is generated across the coil's secondary winding, which ignites (fires) the spark plug and combusts the fuel and air mixture in the cylinder. This sequence of events, which is repeatedly performed during operation of an associated engine, results in significant electrical stress on the components of the ignition circuit.
In a general aspect, an ignition circuit can include a control circuit that is configured to be coupled with an engine control unit (ECU) to receive a command signal from the ECU, and a driving circuit coupled with the control circuit, the driving circuit being configured to be coupled with a resonant circuit that includes a primary winding of an ignition coil. The control circuit and the driving circuit can be configured, in response to a command signal, to drive the resonant circuit at a first frequency to generate a voltage in the ignition coil to initiate a spark in a spark plug coupled with the ignition coil; and, in response to the spark being initiated in the spark plug, drive the resonant circuit at a second frequency to maintain the spark in the spark plug for combustion of a fuel mixture. The control circuit can be further configured to, after the combustion of the fuel mixture, to disable the driving circuit.